1. Field of Invention
The present invention relates to a developing method in an image-forming device. More particularly, the present invention relates to a developing method that uses Image-on-Image (IOI) or Toner-on-Toner forming process in an image-forming device for color electro-photographic printing.
2. Description of Related Art
Electrophotographic systems are generally employed in image-forming devices, the operation of which involves several steps: charging, exposing, developing, transferring, and fusing.
However, when an Image-on-Image (IOI) forming process is used in the image-forming device, the printed image has ghost images and smears thereon.
Take for example the four-pass type IOI color image forming device having yellow, magenta, cyan and black developing units. After the charging, exposing and developing step of the 1st pass, a yellow toner image is formed on the photoreceptor. After the charging step of the 2nd pass, the yellow toner image has another surface potential, which is different from the potential of the magenta developing unit after the developing step of the 2nd pass. Thus, the toners of the magenta developing unit are adhered to the yellow toner image due to the potential difference, and the ghost image is caused thereby. One way to reduce the potential difference between image area and non-image area is to apply erasing step, also called discharge step, at the end of each pass, which intends to bring the photoreceptor back to the neutral state before the next developing pass starts. By bringing down the residual charges, ghosting will be eliminated or reduced.
Furthermore, after the charging, exposing and developing step of each pass, the photoreceptor is discharged for the next pass. Taking the 2nd pass for example, the magenta toner image is formed after the charging, exposing and developing step, and then the photoreceptor is discharged for the 3rd pass. However, the toner-image region and the non-toner-image region on the photoreceptor still have a potential difference therebetween despite that the photoreceptor has been discharged for the 3rd pass, such that the toners of the toner-image region are adhered to the non-toner-image region. FIG. 1 shows the potential difference on the photoreceptor after the discharging step during the 2nd pass in the prior art. At the moment, the absolute value of the potential VS2 of the toner-image region is larger than the absolute value of the potential VSB of the non-toner-image region, so the toners are adhered to the non-toner-image region due to the potential difference, to cause the smear problem.
For the foregoing reasons, there is a need to provide a developing method to solve the problem of low image quality caused by the ghost image and smear at the same time.